In a typical ion implanter, a relatively small cross-section beam of ions containing a desired atomic species is scanned relative to a substrate to be implanted, typically a semi-conductor wafer.
The beam may be scanned transversely in two dimensions relative to the stationary wafer, or the wafer may be scanned in two dimensions relative to a stationary beam. There are also hybrid scanning techniques when the beam is scanned in one dimension whilst the wafer is mechanically scanned in a second typically orthogonal direction.
The various techniques have advantages and disadvantages. For batch processing of semi-conductor wafers, the wafers of a batch can be mounted on a rotating wheel and the axis of rotation of the wheel can then be scanned to and fro to provide two-dimensional mechanical scanning of the wafers across a stationary beam. An example of batch implanter of this kind is described in U.S. Pat. No. 5,389,793.
Single wafer implanters can employ the hybrid mechanical and electrostatic or electromagnetic beam scanning outlined above. Such an arrangement is described in our commonly assigned U.S. Pat. No. 5,898,179. Here, the ion beam is electromagnetically scanned in a first direction perpendicular to the beam axis in the ion implanter, whilst the wafer is mechanically moved in a second generally orthogonal direction.
It is important in implanting to ensure that the total dose of desired species implanted into the semi-conductor wafer or other substrate has the desired level of dose uniformity over the entire substrate surface. In the above-described batch-type of implanters, this is achieved by spinning the implant wheel at high speed and scanning the wheel axis to and fro so that the wafers mounted on the wheel pass across the beam many times during an implant process. In the hybrid single-wafer implanters also mentioned above, dose uniformity is maintained by performing the electrostatic or electromagnetic beam scanning at a relatively high rate compared to the mechanical movement of the wafer to be implanted. Dose uniformity over the wafer surface in the direction of mechanical movement of the wafer is ensured by controlling the rate of this mechanical movement, but the rate of mechanical movement is always much slower than the beam scanning rate.